A category of fluid bearings in which the rotary shaft is supported by the film pressure of a lubricating oil interposed in a clearance between the outer peripheral surface of the shaft and the inner peripheral surface of the sleeve includes the following bearings: a fluid dynamic bearing in which the hydrodynamic developed by a groove provided either on the outer peripheral surface of the shaft or on the inner peripheral surface of the sleeve forms a film of a lubricating oil to raise and support the sliding surface of the rotary shaft; a porous oil-impregnated bearing in which a porous body of a sintered metal and the like is rendered self-lubricating by impregnation with a lubricating oil or grease and supports the rotary shaft; and a hydrodynamic type porous oil-impregnated bearing which is a porous oil-impregnated bearing provided with a groove for developing hydrodynamic on its bearing surface. They are suitable for use in instruments which are required to be driven at high speed with high rotary precision such as spindle motors in polygone scanner motors of laser beam printers (LBP) and in hard disk drives (HDD) and suitable for instruments which are driven at high speed under the conditions of imbalance loading caused by mounting of a disk such as spindle motors in optical disks like DVD-ROM and DVD-RAM or in magneto optical disks like MO.
In order to meet demands for low-torque bearings in small-sized spindle motors for information equipment, lubricating oils of relatively low viscosity are chosen for fluid dynamic bearings, porous oil-impregnated bearings and hydrodynamic type porous oil-impregnated bearings. Low-viscosity lubricating oils proposed hitherto include hydrocarbon-based PAO (poly-α-olefins), the diesters described in JP92-357318 A, the dialkyl carbonates and polyol esters described in JP96-259977 and the monoesters described in JP2000-63860.
Keeping pace with a growing trend in recent years for higher performance and mobile use in audio-visual and automated office equipment, there is a strong demand for greater speed and smaller size for spindle motors to be installed in the rotary parts of the equipment and, in consequence, always a demand for low-torque bearings in the parts supporting rotation. Factors influencing the torque of a bearing include the clearance and diameter of a bearing and also the viscosity of a lubricating oil is one of such factors.
Lubricating oils generally tend to evaporate more easily as the viscosity decreases. They cannot develop an adequate oil film pressure when they suffer loss by evaporation and the like and the rotary precision deteriorates markedly so that the bearing in use is regarded to have come to the end of its life; hence, the evaporation characteristics of lubricating oils are important properties critically influencing the durability of bearings. It is therefore necessary to select lubricating oils of low viscosity and good low volatility characteristics for lubrication of sleeve bearings such as fluid dynamic bearings, porous oil-impregnated bearings and hydrodynamic type porous oil-impregnated bearings. Ester-based lubricating oils are used as such on many occasions.
Ester-based oils are available in several kinds, differing from one another in viscosity, low volatility characteristics, solubility behavior and the like and, similar to other kinds of lubricating oils, they tend to deteriorate in evaporation characteristics as the viscosity decreases. Therefore, in order to reduce the torque of bearings, a mere selection of ester oils which are lower in viscosity than the existing ones may adversely affect the evaporation characteristics and lower the durability of bearings. For example, a mixture of a low-viscosity oil composed mainly of monoesters and a high-viscosity oil seemingly shows a high viscosity and is expected to evaporate to a small extent; however, selective evaporation of low molecular weight ingredients decreases the amount of oil or selective intrusion of low molecular weight ingredients into the lubricating surface decreases the rigidity of bearings. When diesters are used, it is possible to obtain low-viscosity lubricating oils by adequate selection of the molecular weight. In the case of diesters with a viscosity of 10 mm2/s or less at 40° C., however, the amount of evaporation increases as the molecular weight decreases and, at the same time, evaporation occurs nearly all at once because of the uniformity of molecular weight. Thus, the durability deteriorates rapidly when the conditions are off the boundary of the specified ones.